The fire erupted on Wednesday afternoon in a rural area and devoured timber and brush on both sides of Interstate 5 (I-5) as it nearly tripled in size overnight, officials said.

Elsewhere in the state, a fire raging in the Sierra Nevada had grown to more than seven square miles after shutting down stretches of US 395, State Route 108 and the Pacific Crest Trail along the eastern spine of California.

The Marine Corps Mountain Warfare Training Center, campgrounds and other areas were evacuated.

Ranchers were told to prepare to move livestock out of the area in Humboldt-Toiyabe National Forest.

The two fires on Thursday were raging just weeks after a blaze in the Redding area killed eight people and burned some 1100 homes in a frightening start to the fire season.

The Delta Fire burns in the Shasta-Trinity National Forest
Noah Berger/AP

California's insurance commissioner said victims of that fire and one in the Mendocino area - the two largest blazes in the state so far this year - have filed more than 10,000 claims so far totalling 845 million dollars.

The two fires destroyed or damaged a combined 8800 homes and 329 businesses.

"The worst may be yet to come," Commissioner David Jones warned at a San Francisco news conference, noting California wildfires are typically more destructive after September 1.

California Highway Patrol Officer Jason Morton said officials from a number of agencies were meeting to determine if they can reopen the highway, a key route for commercial trucks.

The highway runs north from the Mexico border through California, Oregon and Washington state to the border with Canada.

The scattered homes and cabins in and around Shasta-Trinity National Forest were under evacuation orders, from the community of Lakehead north to the Siskiyou County line, said Chris Losi, a spokesman for the forest.

"It isn't a lot of people," he said.

A scorched logging truck rests on the road after becoming caught in the Delta Fire
AP

The blaze was human-caused, officials said, without indicating whether it was arson or an accident.

About 17 trucks were abandoned along the interstate and at least four caught fire, Lieutenant Commander Kyle Foster of the California Highway Patrol's Mount Shasta office told the Los Angeles Times.

At least two trucks were partially melted.

US Forest Service workers helped the driver of one flaming truck to safety. Truckers, firefighters and others aided more drivers.

Brandon Vaccaro, of the California Department of Forestry and Fire Protection, told the Redding Record Searchlight: "There's vehicles scattered all over.

"Whatever occurred here was probably pretty ugly for a while."

About 45 miles of I-5 were closed in both directions, Mr Losi said.

The blaze also delayed Amtrak's Coast Starlight train service between Sacramento and Oregon.

The city of Dunsmuir, with about 1500 residents, was about 15 miles from the flames.

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